Hitherto, as a method of drying at low temperature a substance susceptible to heat denaturation, some technologies have been proposed.
For example, Patent Literature 1 (JP 2005-287373 A) proposes the following method. Specifically, a depressurized superheated vapor is introduced into the drying apparatus main body kept at reduced pressure, and an object to be dried such as food, which is susceptible to heat denaturation and placed in the drying apparatus main body, is dried by the depressurized superheated vapor.
However, in this method, the placed object to be dried is subjected to contact and heat exchange with the depressurized superheated vapor. Accordingly, when the object to be dried is a liquid raw material, it is difficult to ensure a large evaporating heating surface, and there is a problem in that the apparatus is increased in size in order to ensure the large evaporating heating surface.
Further, Patent Literature 2 (JP 2005-291598 A) proposes the following method. Specifically, fluidized particles such as glass beads, and an object to be dried such as food, which is susceptible to heat denaturation, are charged into the fluidized bed drying apparatus main body, and a depressurized superheated vapor is introduced into the fluidized bed drying apparatus main body at reduced pressure. The object to be dried and the depressurized superheated vapor are brought into contact with each other in a fluidized state. In this manner, the object to be dried is dried.
However, also in this method, it is necessary that the object to be dried be mixed with the fluidized particles in the fluidized bed drying apparatus main body so as to form the fluidized state, and hence this method is inappropriate as a method of drying and powdering a liquid raw material as a substance susceptible to heat denaturation.
Still further, Patent Literature 3 (JP 2006-297243 A) proposes the following vacuum spray dryer. Specifically, liquid to be dried is atomized by the ultrasonic atomizing device. Spray droplets thus generated are introduced into the vacuum chamber, and are heated by the heating means such as a silicone rubber heater fixed to the outer peripheral surface of the vacuum chamber. The spray droplets are heated and dried at reduced pressure, and thus are powdered.
However, in this method, from the heating means fixed to the outer peripheral surface of the vacuum chamber, heat is transferred in a vacuum to the spray droplets introduced into the vacuum chamber. Accordingly, there is a problem in that a heat transfer efficiency is poor.
Still further, Patent Literature 4 (JP 2006-333838 A) proposes the following method. Specifically, using the compressed air, fermented milk is sprayed into the drying tower kept at reduced pressure, and is heated by the far-infrared heater provided to the drying tower. The atomized fermented milk is dried at a temperature of 50° C. or lower at reduced pressure. In this manner, fermented milk powder is produced.
However, in this method, due to lack of quantity of the air that is introduced into the drying tower so as to flow in the drying tower, it is difficult to blow off the fermented milk powder produced by drying. Further, radiation heat from the far-infrared heater is less likely to transfer to the center portion of the inside of the drying tower. Accordingly, there is a problem in that this method is inappropriate for high-volume production in spite of its appropriateness for low-volume production.
Patent Literature 5 (JP 2009-103398 A) proposes the following method. Specifically, raw material liquid such as chemicals, and spraying gas such as nitrogen gas are introduced via the two fluid nozzle into the dryer main body kept at reduced pressure, and the raw material liquid is atomized in the dryer main body. Further, heating gas formed of the air or nitrogen gas is introduced into the dryer main body from the hot air introducing portion provided on the outer peripheral side of the dryer main body. In this manner, the raw material liquid atomized in the dryer main body is dried and powdered by the heating gas.
However, in this method, the heating gas is formed of non-liquefied gas (which is non-condensable) such as the air and nitrogen gas. Accordingly, it is difficult to keep a constant degree of vacuum of the inside of the dryer main body, and a large quantity of gas is required in order to ensure a heat quantity necessary for drying because the heating gas has a relatively small heat capacity. Further, the hot air introducing portion for introducing the heating gas is separated from the two fluid nozzle, and hence heat of the heating gas is not sometimes transferred efficiently to the atomized raw material liquid. Therefore, there is a problem in that the apparatus is increased in size and it is difficult to perform powdering in a short period of time.